A layered circuit board includes a plurality of boards which are stacked. Commonly, conductor pads which are connected with wiring layers are respectively provided to these boards. As a method for layering these boards, such method is widespread that a via is formed in an insulation layer interposed between boards so as to electrically connect conductor pads of boards through the via. Specifically, a plurality of via holes which penetrate in a thickness direction are formed with a laser or a drill, for example, in an adhesive resin sheet made of prepreg or the like. Then, these via holes are filled with conductive paste and the adhesive resin sheet filled with the conductive paste is interposed between a first board and a second board. Subsequently, processing of pressurizing in a layering direction under heating, referred to below as “heat press treatment”, is performed. Accordingly, the conductor pads respectively formed in the first board and the second board are electrically connected with each other through a via and the first board and the second board are layered on each other.
FIG. 21 illustrates a layered circuit board of related art. As depicted in FIG. 21, an adhesive resin sheet 3 of which a via hole is filled with conductive paste 2 is used in a layered circuit board of related art. This adhesive resin sheet 3 is interposed between a pair of boards 4. FIG. 21 illustrates a state before heat press treatment is performed. Conductor pads 6 which are respectively formed on bonding faces 5 of the boards 4 are protruded from the bonding faces 5 by the thickness thereof. Therefore, an inter-conductor gap 7 is formed between the conductor pads 6 which are adjacent to each other in a planar direction (in-plane direction) of the boards 4.
The inter-conductor gap 7 functions as a space to which softened resin of the adhesive resin sheet 3 may move when the heat press treatment is performed. Thus, resin contained in the adhesive resin sheet 3 is softened to flow into the inter-conductor gap 7 in the heat press treatment. Accordingly, the adhesive resin sheet 3 is easily deformed. As a result, metal particles contained in the conductive paste 2 are easily pressurized, improving an aggregation state of the metal particles.
Japanese Laid-open Patent Publication No. 1995-176846, Japanese Laid-open Patent Publication No. 1987-243395, Japanese Laid-open Patent Publication No. 2005-310871, and Japanese Laid-open Patent Publication No. 2007-335701 are examples of related art.